Hydraulic Operation

INDIVIDUAL PRESSURE MODULATION
ABS/TC CM hydraulic modules incorporate inlet valves (items 16 and 17) and outlet valves (items 20 and 21) for each driven wheel. This enables individual pressure modulation to the rear brakes under wheel spin conditions, i.e. traction control.

Increased Wheel Spin - of a driven wheel under acceleration causes the NO isolation valve (item 14) to be closed and the pump (item 10) to be switched on. This is in response to signals from the control module.

Closing Of The Isolation Valve Blocks - delivery of the pump to the secondary circuit (item 2) of the TMC. The pump now draws fluid from the reservoir via the open hydraulically operated inlet valve (item 13). Increased pressure is now available at the inlet valves (items 16 and 17) for actuating the rear brakes, thus decreasing the tendency of wheel spin.

NOTE: The hydraulic inlet valve (item 13) switches When traction control is initiated to change the suction connection of the pump from the accumulators (item 9) to the fluid reservoir (item 6) via the TMC.

The Pressure At The Inlet Valves - corresponds to the opening pressure of the relief valve (item 15) incorporated in the isolation valve. Excess brake fluid is drained to the suction side of the pump via the relief valve and returns either to the TMC secondary circuit and on to the fluid reservoir, or is directly drawn on by the pump.

As The Spinning Wheel Has Been Braked - down into the normal range of wheel spin, the NO valves (items 16 or 17) close to prevent any further increase in brake pressure. Depending upon the acceleration of the wheel, the NC valve (item 20 or 21) may open to decrease the secondary circuit brake pressure. NC valves (item 17 or 18) may remain closed in order to achieve a brake pressure holding phase. If the pressure in the secondary circuit needs to be increased again, the NC valve closes again (if open) and the NO valve opens, diverting the necessary volume flow. This control action, keeps the wheel in the range of optimum slip until the spinning tendency ceases. The isolation valve (14) remains closed throughout the traction control cycle.

Actuation Of The Brake By The Driver - sensed by the control module, causes the traction control mode to be terminated and the isolation valve (item 14) to be opened. The TMC pressure simultaneously closes the hydraulic inlet valve (item 13) so that the pump can no longer draw fluid from it. The ABS/TC CM now operates in normal ABS mode.

NOTE: When traction control is initiated, speed control is de-activated (if in operation) and requires re-setting after the traction control mode has terminated. Gear shift is inhibited on automatic transmission vehicles; no downshifts are allowed and upshifts occur at 4800 RPM.